I have no heat. on 2001 GMC Sierra 1500

I have no heat. The blower works fine. It just blows cold air. I used the air this problem. I don't know when I last used the heater. I haven't had any problems with it until yesterday. Appreciate any suggestions.

Asked by for the 2001 GMC Sierra 1500
Two areas to look at -- if the engine is getting hot enough, and if the doors in the dash that control the temperature are working as they should. Start by checking the temperature of the hose going into and coming out of the heater core. Are they both blistering hot - in and out? If so, then start looking inside for the temperature blend issue. If they are not, you need to get them hot first, then recheck from there. A new thermostat or further cooling systtem repairs may be in order. Good luck!
2 more answers , 1 more comment
could be low on coolant or heater core could be pluged
I had a Ford Ranger that ran low on rad fluid and started blowing cold air. Overflow was full and no leaks in it. But it slowly cooled down. Yours sounds more sudden. Hope it helps. I drive a GMC now !